[20250725] BOJ / G4 / 합이 0 / 이인희

LiiNi-coder/202507/25 BOJ 합이 0.md

@@ -0,0 +1,69 @@
+```java
+import java.io.BufferedReader;
+import java.io.InputStreamReader;
+import java.util.Arrays;
+import java.util.StringTokenizer;
+
+public class Main {
+	private static BufferedReader br;
+	private static int n;
+	private static int[] arr;
+
+	public static void main(String[] args) throws Exception {
+		br = new BufferedReader(new InputStreamReader(System.in));
+		n = Integer.parseInt(br.readLine());
+
+		arr = new int[n];
+		StringTokenizer st = new StringTokenizer(br.readLine());
+		for (int i = 0; i < n; i++) {
+			arr[i] = Integer.parseInt(st.nextToken());
+		}
+
+		Arrays.sort(arr);
+
+		long count = 0;
+
+		for (int i = 0; i < n - 2; i++) {
+			int fixed = arr[i];
+			int left = i + 1;
+			int right = n - 1;
+
+			while (left < right) {
+				int sum = fixed + arr[left] + arr[right];
+
+				if (sum == 0) {
+					if (arr[left] == arr[right]) {
+						int length = right - left + 1;
+						count += (long) length * (length - 1) / 2;
+						break;
+					}
+
+					int leftVal = arr[left];
+					int rightVal = arr[right];
+					int leftCnt = 0;
+					int rightCnt = 0;
+
+					while (left <= right && arr[left] == leftVal) {
+						leftCnt++;
+						left++;
+					}
+
+					while (left <= right && arr[right] == rightVal) {
+						rightCnt++;
+						right--;
+					}
+
+					count += (long) leftCnt * rightCnt;
+
+				} else if (sum < 0) {
+					left++;
+				} else {
+					right--;
+				}
+			}
+		}
+
+		System.out.println(count);
+	}
+}
+```